Burton's old Theatre, formerly dedicated to the 'sock and buskin,' and famous during the religious revival of 1858, was now occupied by this convention of marshals. Waiving unnecessary parliamentary usages, these ministers of the law sat with closed doors, and discussed familiarly the business in which they had engaged. They investigated carefully the whole subject in its minuter details, and visited the slave brigs and schooners which had been captured and were then lying at the Atlantic Dock in Brooklyn. A plan of operations was concerted, by which the marshals of the different districts should co-operate with each other in detecting and bringing to justice persons guilty of participating in the slave-trade. The results of this measure can not fail to be beneficial; and, indeed, the marshals have already become so active and efficient, that the capitalists who have maintained this branch of commerce are actually contemplating its transferment to European ports. So much for the convocation at Burton's Theatre. Let us now examine the principal features of the traffic, and the practices of those by whom it is conducted. SLAVE DEALING IN NEW YORK. The principal slave captains and chief officers of vessels engaged in the slave-trade have their residences and boarding-places in the eastern wards of the city, most of them being between James and Houston Streets. They are known to every one who has an investment in the business. Indeed, they are all members of a secret fraternity, having its signs, grips, and pass-words. 'While I was in Eldridge-street jail,' said one of them, 'Captain Loretti was captured and brought there. He did not know any one, but I shook hands with him, and we became acquainted at once.' The arrival of a slave captain from one voyage is the signal for preparation for another. Negotiations are carried on, generally in the first-class hotels. The contracts for the City of Norfolk and several other notorious slavers were made at the Astor House. The risk of detection is less at such a public place than it would be at a private office. A man who had failed in business on Greenwich Street was recently engaged in fitting out these vessels for their African voyage. He was first sent to procure apparatus for the refining of palm oil. This was but a blind, the practice being to take out the machinery, and employ the boiler for culinary purposes, until the vessels had got out to sea, and there was no farther necessity for duping inquisitive persons. This man was also commissioned to purchase wooden ware, champagne, and other necessary articles. Such were the business agents and their duty; all was liberally paid for and promptly supplied.